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Why Aquarium Plants Fail


by Neale Monks


If you're having problems with your aquarium plants, the chances are that the solutions are simple to understand. Lack of adequate lighting is by far the most common problem, but other important issues to consider include water chemistry, substrate type, and the way each plant species is installed into the aquarium.

Contrary to what many suppose, mineral nutrients like iron and carbon dioxide aren't make-or-break issues unless you're keeping the more difficult plant species. If you choose hardy plants, you won't need to worry about carbon dioxide at all, and iron supplements can be added to the aquarium water in just the same way as you dose water conditioner.

In this article we'll look at the most common reasons aquarium plants fail and what you can do to keep your tank looking lush and green.


Above all else, plants need good lighting. However, the sad truth is that many off-the-shelf aquarium starter kits contain rather weak lighting, and except perhaps for Java ferns and Anubias, nothing much will grow in them.

So how much light do you need? This is really difficult to explain in simple terms because the unit of light intensity, lux, isn't one that most aquarists are familiar with, and fewer still will have any way of measuring it.

Instead, aquarists often rely on a 'watts per gallon' measurement that provides a very approximate idea of how much light there is in an aquarium.

The problem is that the watts-per-gallon rule doesn't work well for small tanks or large tanks. In very small tanks the tube will be so short that it won't be providing much light energy per square inch of aquarium floor. That's because small tanks tend to be proportionally rather deep compared to their length, and standard fluorescent tubes provide light energy in proportion to their length. So each inch of fluorescent tube in a small tank will be effectively lighting up more gallons of water than would be the case for a fluorescent tube fitted to a large tank. In very large tanks standard fluorescent tubes don't produce the light intensity needed to push light energy down to the very bottom of the tank in sufficient amounts to support low-growing plants.

The bottom line is that the watts-per-gallon rule works best for tanks between about 20 gallons and 100 gallons, at least as far as standard T5 and T8 fluorescent tubes go.

Aquarists with tanks smaller than 20 gallons or bigger than 100 gallons will need to provide better, stronger lighting than fluorescent tubes can provided. High-performance compact fluorescent tubes can provide a low-cost option for undemanding plants in small tanks, and metal-halide lamps do work well over large tanks, but high-output LED lamps are increasingly widely sold and work extremely well in both these demanding situations.

There are really only two plant types that can do well with 1 watt per gallon or even slightly less, Java ferns and Anubias species. Both of these plants are epiphytes, meaning they grow attached to something rather than in the substrate. Under aquarium conditions they're usually grown attached to bogwood roots or lumps of lava rock. Java ferns and Anubias species are very adaptable plants that do well across a range of water chemistry conditions, including brackish water. They are comparatively expensive though, and this often puts newcomers off buying them. That's a mistake, because kept properly, they're among the most reliable plants around and healthy specimens can live for many years, even decades, slowly spreading out and yielding many daughter plants or cuttings.

Between 1 and 2 watts per gallon your variety of plant species opens up considerably. As well as Java ferns and Anubias species, you can add to your planting list two wonderful floating plants, Indian Fern (Ceratopteris thalictroides, also known as Water Sprite) and Amazon Frogbit (Limnobium laevigatum). These provide welcome shade and soak up nitrate, reducing algae problems. The hardier Vallisneria species can do well under this level of lighting too, particularly Vallisneria americana (Giant Vallisneria) and Vallisneria spiralis (Common Vallisneria). Vallisneria are very good plants for use along the back of the tank. Among the Amazon Swords, the commonly traded species Echinodorus bleheri will do well at around 1.5-2 watts-per-gallon. Cryptocoryne wendtii and Cryptocoryne beckettii, are particularly good Cryptocoryne species for tanks with this level of lighting and can be very useful for filling the middle part of the tank. The Onion Plant, Crinum thaianum, is a big, slow-growing plant that can do well with around 1.5-2 watts-per-gallon.

With 2-3 watts-per-gallon your range of plants expands further to include many of the popular stem plants such as Hygrophila polysperma and Bacopa monnieri. The thing about stem plants is that they may grow with less light than this, but rarely look good. Rather than growing into nice bushy plants, they instead 'bolt' up towards the light, producing long stems with small leaves, and then produce most of their foliage at the top of the tank. This isn't an attractive look, and such plants tend to fall apart after a few months anyway. Other moderately demanding plants include Aponogeton species such as Aponogeton crispus, green Cabomba species like Cabomba caroliniana, Crinum calamistratum, the Dwarf Amazon Sword Echinodorus tenellus, and the recently discovered but increasingly popular little plant Pogostemon helferi.

Finally, with 3 watts or more per gallon, your choice of plants expands to include species with red leaves such as Rotala indica and Alternanthera reineckii. Red-leaved plants are almost always adapted to habitats with very bright light, and such plants will not grow unless given lots of light. They also tend to be fussier about other aspects of their care as well, particularly carbon dioxide fertilisation. Other plants that need strong lighting include Pennywort (Hydrocotoyle leucocephala) and red Cabomba species such as Cabomba furcata.


Aquarium plants can be divided into three basic types: (1) plants that float; (2) plants that grow attached to wood or rocks; and (3) plants that grow with their roots in the substrate. The first two groups couldn't care less about the substrate used, and for this reason are especially versatile plants and ideally suited to tanks with burrowing fish likely to upset rooted plants.

When it comes to choosing the right substrate for rooted plants, there's much discussion over what's best but little hard evidence either way. True aquatic plants (as opposed to swamp plants) can absorb most if not all of the nutrients they need through their leaves, so the type of substrate needed isn't particularly important. So long as aquarium fertiliser is added to the water periodically, such plants can live perfectly well rooted in plain gravel or sand.

True aquatic plants are those species that live permanently submerged, and quickly die if exposed to the air. Commonly grown examples include Cabomba, Elodea, Myriophyllum and Vallisneria. Such plants have soft leaves and floppy stems, and collapse when pulled out of the water.

Most of the other plants aquarists grow are actually adapted to spending part of the year above the waterline, for example during the dry season. These include such things as Aponogeton, Cryptocoryne, Echinodorus (Amazon Swords), Hygrophila, Rotala, and Sagittaria. When removed from the water these plants have stems that hold themselves up against gravity, something true aquatic plants generally can't do. In some cases these plants grow one sort of leaf when they're underwater, and another sort of leaf when they're out of the water. Sagittaria show this particularly well, with tape-like leaves below the waterline and arrow-shaped leaves above the waterline.

All these plants tend to be more dependent on their roots than true aquatic plants. While they may do okay in plain gravel if fertiliser is added to the water, they often do even better when 'fed' through their roots as well. If grown in plain gravel or sand, then nutrients can be added in the form of fertiliser tablets pushed into the area around their roots. Alternatively, a nutrient-rich substrate can be used right from the start, and the plants grown in that. This is the approach often taken by aquarists designing tanks where rich plant growth is important, for example, Amano-style aquaria. But casual aquarists can get good results with a combination of gravel and fertiliser tablets, assuming all other factors, such as lighting, are appropriate to the plants being grown.

Carbon dioxide

The use of carbon dioxide to increase plant growth is now well established in the hobby. It is most important when growing fast-growing species under high intensity lighting. If you want an Amano-style aquarium, then carbon dioxide fertilisation will probably be essential. This is an expensive proposition though, and carbon dioxide fertilisation is fiddly to do at the best of times, and without careful planning can waste money and risk the lives of your fish. There's an ample literature available on growing plants in aquaria using carbon dioxide fertilisation, but the basics are covered elsewhere on WWM .

Casual aquarists don't need to worry about carbon dioxide though. The plants listed above that grow more slowly and are happy at 1-3 watts-per-gallon are all species that will get by just fine without carbon dioxide fertilisation.

Floating plants and aquarium hoods

There are several floating plants traded, including Amazon Frogbit (Limnobium laevigatum), Crystalwort (Riccia fluitans), Indian Fern, also known as Water Sprite (Ceratopteris thalictroides), Water Cabbage or Water Lettuce, Pistia stratiotes, and various Salvinia species.

The main problem with floating plants is that the heat underneath lights can burn them. Pistia and Salvinia are particularly affected by this and normally only do well in open-topped tanks where the lights are held a long way above the waterline. Amazon Frogbit grows more or less flat on the surface so doesn't get easily burned so is a good choice for tanks with hoods. Crystalwort and Indian Fern mostly grow under the waterline but may form clumps above the waterline. Such growth can be cut away if it shows signs of burning without harming the main body of the plant. As such, they're also good choices for tanks with hoods.

Duckweed (Lemna minor) is another floating plant usually considered a pest rather than a decorative plant. Nonetheless this species is easy to grow and provides useful shade in very small tanks, so can be useful. On the other hand, it grows very quickly, so if you don't want this plant in your tank, be sure to remove them on sight otherwise they'll quickly multiply and make a nuisance of themselves.

Non-aquatic plants

Unfortunately there are a lot of plants sold in aquarium shops and pet stores that will never live long in aquaria. These non-aquatic plants are discussed elsewhere on WWM They are not worth spending money on, and if you already have some, then remove them and treat them as you would a houseplant. Indeed, many of these plants are houseplants, and details on their proper care can be found in books about growing plants indoors.

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